The present invention relates to seismic exploration and more particularly to a method and apparatus for preferentially detecting seismic shear waves having a given direction of propagation. In the above referenced patent, there is described an apparatus and method for preferentially detecting seismic compressional waves having a given direction of travel. The term "direction of travel" is used to describe whether the seismic wave propagating upwardly or downwardly through the earth. As explained in the patent, the preferential detection of a seismic wave is important in correcting the seismic data for various errors and removing unwanted or noise signals from the seismic data.
Recently, there has been renewed interest in utilizing shear waves for obtaining seismic data. The use of shear waves for obtaining seismic data in contrast to the use of compressional waves permits one to obtain information regarding some subsurface formations that is impossible to obtain from compressional waves. While means are known for preferentially detecting compressional waves, no methods or apparatus have been devised as yet for preferentially detecting shear waves traveling in a given direction. Preferential detection of shear waves is necessary in order to remove unwanted noise from seismic data, particularly when the detector is placed below the surface of the earth.
Compressional (dilatational) waves are defined as waves in which the direction of particle displacement in a medium is normal to the wave front while shear (rotational) waves are defined as waves in which the direction of particle displacement in a medium is parallel to the wave front. While a conventional velocity geophone can be disposed to detect shear waves, it will not discriminate waves based on their direction of travel. Likewise the method disclosed in the above referenced patent cannot be used to preferentially detect shear waves. The patent discloses the use of a velocity geophone and hydrophone in combination to preferentially detect compressional waves having a given direction of travel. The hydrophone responds to the pressure fluctuation produced by the particle movement while the geophone responds to the particle movement. In the case where the hydrophone signal remains the same for both uptravelling waves and down-travelling waves, for example, the geophone signal reverses polarity as the direction of travel of the waves reverses. It is then possible to preferentially detect up- or down-travelling waves by adding or subtracting the two signals. This method cannot be used to preferentially detect shear waves since no dilatation (pressure fluctuation) is associated with the shear wave. Thus, if the geophone and hydrophone were properly disposed, the method would detect the particle motion associated with the shear wave, parallel to the wave front, but not discriminate the direction of travel of the wave front.